In an imaging apparatus for taking an image of a subject, various methods for suppressing an influence of flicker are studied. The flicker here means a flickering phenomenon in alternating current lighting illumination or the like. For example, in a general fluorescent lamp, blinking is repeated at a frequency twice a power supply frequency. In a case of an alternating current power supply of 50 Hz, a blinking frequency of a light source is 100 Hz and a blinking period is 1/100 second. In a case of an alternating current power supply of 60 Hz, the blinking frequency of the light source is 120 Hz, and the blinking period is 1/120 second. The blinking frequency is generally so high that it can not be grasped by the naked eye. However, when an imaging apparatus is used and takes a picture of a subject illuminated by a light source at a shutter speed higher than the blinking speed, the flicker is recognized.
On the other hand, a method is known which prevents flicker at the time of imaging by causing the shutter speed of the imaging apparatus to become a multiple of the blinking period of the light source, and is widely applied to products. That is, in a case of a general fluorescent lamp, when the power supply frequency is 50 Hz, the blinking period of the light source is 1/100 second. Thus, the shutter speed is made 1/100 second or 1/50 second. Besides, when the power supply frequency is 60 Hz, the blinking period of the light source is 1/120 second. Thus, the shutter speed is made 1/120 second or 1/60 second.
Besides, as a related art method, for example, patent literature 1 discloses a method in which the luminance of a subject is measured at the time of slit exposure (including the time of continuous shooting mode), a flicker period and a flicker phase are calculated, and an exposure start timing and a timing when the luminance of the subject becomes maximum are synchronized based on these, so that the influence of flicker becomes as small as possible.
Besides, patent literature 2 discloses a method in which a timing when the brightness of a subject becomes maximum and a flicker period are detected, reading of an image signal from an imaging element is controlled so that the timing when the brightness becomes maximum coincides with an almost center of an exposure period of one frame of the imaging element, and the frame period becomes equal to the flicker period, and shading is suppressed.    Patent literature 1: JP-A-2010-74484    Patent literature 2: JP-A-2009-77057
As described above, when the blinking period of the light source, that is, the power supply frequency is known, and it is known that the light source frequency is a multiple of the power supply frequency as in the general fluorescent lamp, the flicker can be prevented by causing the shutter speed to become a multiple of the light source blinking period. Besides, when the light source frequency (power supply frequency) is not known, as in patent literature 1 or patent literature 2, measures can be taken by detecting the flicker period and the flicker phase.
However, if a subject is illuminated by plural light sources having different light source frequencies, the flicker can not be prevented by the foregoing various techniques. For example, there is a case where a subject is illuminated by a fluorescent lamp having a light source frequency of 100 Hz and a fluorescent lamp having a light source frequency of 120 Hz. In this case, when the shutter speed of the imaging apparatus is made 1/100 second, a flicker of 120 Hz is recognized. When the shutter speed is made 1/120 second, a flicker of 100 Hz is recognized.
In this case, the flicker can be prevented by prolonging the shutter speed so that the shutter speed becomes a multiple of the blinking period common to the plural light sources, here, 1/20 second. However, when the shutter speed is made long, there occur such defects that an afterimage is generated in the moving subject, an image becomes bright by the increase of the exposure time, and an image desired by the user can not be obtained.